Process for avoiding cracking in welding

ABSTRACT

The new welding process avoids cracking in welding, in repair welding or in cladding of parts of metallic alloys which are sensitive to hot cracking. The process is using a first heat source ( 15 ), directed to the parts ( 11, 12 ) of the metallic alloy forming a melt pool ( 14 ) on the parts ( 11, 12 ) of metal or metallic alloy. The heat source ( 15 ) and the parts ( 11, 12 ) are moved relative to each other. The process is characterized in that there is one ( 13 ) or more additional heat sources directed to the parts ( 11, 12 ) of metal or metallic alloy and following the first heat source ( 15 ) in a distance and with substantially the same speed and in the same direction as the first heat source ( 15 ). The additional heat source ( 13 ) or heat sources are directed to the solidification region (mushy zone) ( 144 ) of the melt pool ( 14 ) generated by the first heat source ( 15 ). The power of the additional heat source ( 13 ) is set such as to reduce the local cooling rate of the solidification region ( 144 ) of the melt pool ( 14 ), or to even shortly reheat this region without substantial remelting or with no remelting it at all and thereby reducing the tensile stresses or even inducing compressive stresses. During this process a central equiaxed zone might also be enhanced. By this new process the formation of hot cracks is avoided.

The invention is related to a welding, repair welding or claddingprocess of metallic alloys according to the preamble of the independentclaim 1. It is further related to the use of the welding, repair weldingor cladding process and to work pieces welded or clad with the process.Its main aim is the prevention of hot-crack formation during theprocess.

The general behavior of solidification and hot cracking (solidificationcracking) is very similar in the three processes mentioned. It isfurther general to many metallic alloys like steels, super alloys,aluminium alloys. Therefore only the welding process will be explainedin more detail, with the aid of hot cracking of prone aluminium alloys.Aluminium alloys are traditional materials in transport technology suchas aerospace, automobiles and trains, because of a good combination ofmechanical properties and low weight.

Different joining techniques are used for producing aluminium parts.Riveting, TIG and MIG welding are traditional processes in themanufacturing industry although they present some important weaknesses.Rivets form weak joints and are especially vulnerable to stresscorrosion cracking. TIG and MIG produce large heat-affected zones (HAZ),where alloys experience additional solution treatments and averaging,thus leading to a degradation of material properties and a reduction inlifetime. Laser welding is a particularly interesting approach for theconstruction of metallic structures. New developments in lasertechnology, such as fiber-optic delivery of YAG beam and high-powerdiode lasers, have increased and will increase in the future their usein high volume production.

Many aluminium alloys are weldable provided the solidification intervalis relatively small. Some classes of aluminium alloys such as 2xxx(Al—Cu), 5xxx (Al—Mg), 6xxx (Al—Mg—Si) and 7xxx (Al—Zn— . . . ) oftencrack during autogenous welding. Industrial experience has shown thathot cracking can be avoided by the addition of a eutectic-forming alloy,such as a Al—Si wire, to the weld. This methodology is widely applied tothe construction of aluminium parts, even if the mechanical propertiesof the weld are not as good as those of the base material.

The term “hot cracking” is used to denote brittleness at temperaturesabove the solidification end (often the eutectic temperature) which isdue to the presence of residual liquid films in-between the dendriticgrains of the solidifying alloy. Materials in which such cracking occursinvariably possess a large solidification interval, since pure metalsand eutectic alloys are not susceptible to hot cracking. During coolingof these alloys from the liquid, the formation of primary dendritesbegins close to liquidus temperature, and during subsequent coolingthese dendrites grow at the expense of the liquid. When the proportionof liquid is still large, the alloys have essentially the properties ofa liquid. Later in the process of solidification, however, the dendritesinterlock and form a coherent network with the remaining liquidoccupying the interstices. During the formation of this network, thereis a progressive increase in strength. However, if the nearly-completelysolidified alloy has a high-strength in compression, it is still weakwith respect to the transmission of shear/tensile stresses as long asinter-dendritic and inter-granular liquid films are present. In parallelto this, the interdendritic liquid experiences an increasing difficultyto flow through the high tortuosity paths in order to compensateshrinkage and deformation of the solid skeleton. The combination ofshrinkage and shear/tensile stresses will therefore lead tounderpressure in the remaining liquid, and thus finally tosolidification cracking. The alloy is therefore susceptible to crackingwhile it is in the brittle temperature range (BTR), i.e. at atemperature corresponding approximately to the last 10% of liquid. Theinvention is related to an improved welding process that overcomes theproblem of cracking and in particular of hot cracking.

The process according to the invention is characterized by the featuresof the characterizing part of the independent claim 1. The dependingclaims are related to favorable improvements of the invention. Theprocess provides for crack free welding of work pieces and in particularmetal sheets.

The invention and the prior art are illustrated and explained in detailswith reference to the pictures and drawings.

The figures show the following:

FIG. 1 a show a schematic, perspective view of two work pieces that arewelded according to the invention,

FIG. 1 b is a schematic side view of the welding area and asolidification profile of the different solid fractions during thesolidification of a dendritic network of the welded area of a workpiece.

FIG. 1 c is a schematic representation of dendritic solidification withassociated solid fraction as a function of distance and phase diagram.

FIG. 2 a shows a schematic side view of the welding setup;

FIG. 2 b is a schematic side view of the laser setup and one possibleconfiguration of a gas supply and gas suction nozzles of the weldingsetup.

FIGS. 3 a and 3 b show the pictures of two sheets welded with a CO₂laser according to a prior art method (FIG. 4 a) and two sheets weldedwith the process according to the invention (FIG. 4 b);

FIG. 4 is an example of a temperature-time curve measured by athermocouple placed close to the weld trace for welding processaccording to the prior art FIG. 4 a and the temperature-time curve ofthe welding process according to the present invention FIG. 4 b;

FIG. 5 is the picture of a specimen welded in the lower part with aconventional welding process and in the upper part with the processaccording to the invention, showing crack healing over the transientzone for an overlapped joint.

On the mesoscale, hot cracks can be distinguished from other cracksformed at distinctly lower temperature by detached grains and cracksurfaces decorated by dendrites. The residual liquid remains on bothfractured surfaces, which sometimes shows a eutectic layer. A few spikesresulting from the opening of inter-granular grain boundaries are alsocharacteristic of hot-cracked surfaces. In most cases, these effects canonly be perceived with a scanning electron microscope.

A possible arrangement for practicing welding process is shownschematically in FIG. 1 a and FIG. 1 b, as well as in FIG. 2 a and FIG.2 b. The two metal sheets 11 and 12 are arranged next to each other,thereby forming a gap 10 The metal sheets 11 and 12 are moved in thedirection of arrow A. The laser beam 15 of a CO₂ laser is directed tothe surface are of the two sheets 11 and 12, and bridging the gap 10.The laser beam 15 is meting the two sheets 11 and 12 and forms a meltpool 14. A second laser beam 13 of a YAG laser is directed to the mushyzone 144 region of the melt pool 14. When the sheets 11 and 12 are movedin the direction of arrow A, the laser beam 13 is following the laserbeam 15. It would of course also be possible that the laser beams 15 and13 are moved in stead of the two sheets 11 and 12. In this case thelaser beams 15 and 13 would be moved in the direction opposite to thedirection indicated by arrow A. In another arrangement both, the sheets11 and 12 as well as the laser beams 11 and 12 can be moved relative toone another. As can be seen there is no overlap of the spots of theenergy sources 15 and 13 on the sheets.

FIG. 2 illustrates in more detail and schematically dendriticsolidification with associated solid fraction as a function of distanceand the phase diagram. In this example the mushy zone 144 is also namedthe dendritic solidification region or zone, where the f_(s) the solidfraction is 0<f_(s)<1. This means that in the mushy zone/dendriticsolidification region/zone there is dendritic solid material, but thatthere is still liquid material in this zone. 0<x %<100% of the materialis still in the liquid phase.

At the temperature T_(t) solidification of the material starts anddendrites start building up. At the temperature T₁ lower than T_(t),down to the temperature T₂ there is the zone of the interdendritic film.In the shown example in this area the solidification factor is0.6<f_(x)<0.9.

When in this application it is stated that the wording “or even shortlyreheat this region without substantially remelting” is used it is meant,that the temperature of the part of the mushy zone exposed to the secondheat source will not go up to T_(t). As a consequence of this, therewill always be dendrites. Only the percentage of liquid will be increaseby such “non substantial remelting” and there will always be soliddendrite material in the mushy zone 144.

As can bee seen in FIG. 2, the process is performed under gasprotection. The gas supply nozzle G supplies the inert gas and the gassuction nozzle S sucks the gas, so that the melt pool 14 is wellprotected by the gas flowing from nozzle G to nozzle S. In the enlargedpart of the sheets 11 and 12 of FIG. 2 b can be seen, that the sheets 11and 12 are arranged overlapping each other in the area that is welded.

The process may also be performed with lasers beams of the same type. Insuch an arrangement the laser beams may come from two separate lasersources or the laser beams laser beams.

Based on present experience of hot cracking phenomena in alloys, theconditions for avoiding cracks can be analyzed. Under normal weldingconditions, the transverse stress distribution near the melt pool alongthe weld centerline consists of three typical regimes. Firstly,compression forces are observed ahead of the melt pool due to heatingand thermal expansion of the solid. Secondly, liquid formation with afree surface accommodates the stresses. Thirdly, tensile forces build upas soon as the mushy zone begins to behave as a continuous solid. Thesetensile forces, which can result in final deformation of the welded partand/or in residual stresses, are often responsible for hot cracks.

The control of process conditions, such as the geometry of the weld, theclamping of the parts, and the laser power and speed could reducestresses behind the melt pool. For example, the clamping distancedirectly influences stresses. For an edge-mounted sample, a smallclamping distance decreases tensile stresses because of the expansion ofthe sheet. If the thermal conductivity and interaction time aresufficiently large this effect leads to compression of the mushy zoneand prevents cracking. However, if the sheets are overlapped this effectleads to sliding, thereby producing cracks in the interface between thesheets. Usually this occurs after complete solidification thus producingcold cracks. Reducing welding heat input and speed also decreases thetransverse stresses, increasing the resistance against cracking.

Rappaz et al. (M. Rappaz, J.-M. Drezet and M. Gremaud, Met. Trans. 30A(1999) 449) assessed the influence of stain rate on the HCS (HotCracking Susceptibility); their model is based on the maximumtensile/shear strain rate which can be supported by the mushy zonebefore cracks appear. The stain rate can be decreased if the coolingrate during solidification is reduced, i.e. if the solidification speedand/or the thermal gradient are reduced. Some thermal gradient controlis possible with preheating, but this cannot always be applied inindustrial manufacturing.

The addition of a eutectic-forming alloy to the weld is recommended asit increases the permeability of the mushy zone in the regions whereshrinkage and stresses occur. For example, the addition of a 4043 Al—Sialloy wire to the 6061 alloy weld reduces the hot crackingsusceptibility. However both the yield and ultimate strengths arereduced by 50%. Furthermore, there might be variable heat inputresulting from the occasional feeding of filler wire directly into thebeam, causing inconsistent penetration and weld-pool instability.

The weld microstructure also plays a role in hot cracking. It isessentially controlled by the growth speed V and the thermal gradient Gat the solidification front, but also by the inoculation conditions. Forcolumnar structures growing from the edge of the weld microsegregationusually produces a centreline channel which is the last part tosolidify, and which is especially sensitive to cracking. Two mechanismscan decrease the HCS of this centreline boundary: formation of equiaxedgrains and a variation in grain orientation.

It has been observed that fine equiaxed grains are less susceptible tohot cracking than columnar grains because the strains are more evenlydistributed among numerous grain boundaries. Another possible techniquefor avoiding crack formation consists of changing the directionality ofgrain growth towards the weld centreline by producing a tortuous path.

The laser welding process according to the invention in principleincludes a precisely controlled cooling cycle and associated stressbuild-up evolution. This is achieved by the combination of two or moreheat sources such as laser beams (FIG. 1). Positioning the laser sourceover the sensitive region of the melt pool which is the mushy zone, thisresults in

-   -   a reduced contraction or even in a compression of this zone;    -   a decreased strain rate. The strain rate can be decreased by a        decrease of the cooling rate during solidification, especially        at high volume fraction of solid;    -   an increased intergranular and interdendritic feeding time;    -   an enhancement of equiaxed grains. An equiaxed structure can be        produced by decreasing the thermal gradient and by producing        mechanical stirring due to a pulsed laser, which may lead to an        additional fragmentation of dendrites.

In the following the invention is explained in detail with the aid of anexample. A 6016 aluminium alloy (Table 1) which is used in automobileswas chosen for the experiments since it is susceptible to hot cracking.FIG. 2 shows a schematic representation of the welding setup, showing inFIG. 2 a the fixture system and the sheets and in FIG. 2 b the setupwith the CO₂ laser beam and the YAG laser beam, together with the gassupply nozzle G and the gas suction nozzle S. The alloy was delivered ina T6 condition, after solution heat-treatment at 540° C. for a shorttime, air cooling, and a precipitation (aging) treatment at 205° C. forseveral hours. The sample dimensions were 100×50×1 mm sheets which werewelded with an overlap of 8 mm FIG. 2 a This geometry is usually used intransportation industry because mounting of butt plates easily leads touncontrolled gaps. TABLE 1 6016 alloy composition in wt. % Si Fe Cu MnMg Cr Ni Zn Ti Al 1.11 0.24 0.07 0.06 0.41 0.013 0.0052 0.015 0.012 bal.

The laser workstation consisted of two lasers, one CO₂ and one YAGlaser, a CNC controlled table (with linear scanning velocities up to 0.5m/s) and a gas protection system, FIG. 2 b. The 1.7 kW CW—CO₂ laserproduced a minimum focal spot of about 0.26 mm diameter with an off-axisparabolic mirror with 152 mm focal length. The 1.2 kW pulsed mode YAGlaser produced a 0.6 mm focal spot given by the diameter of the opticalfibre. For the present application, the mean spot size of the secondlaser was defocused giving an elliptical 1.2×1.5 mm spot with the longeraxis aligned in the direction of the laser movement

Inert gas was applied through a nozzle and evacuated through a suctionsystem to direct the gas stream and protect the optics. This suctionsystem also moved the plasma plume away from the second laser spotallowing a free interaction of the second laser beam with the samplesurface without plasma formation. Pure helium was found to be betterthan argon, or a mixture of Ar/He, since the plasma was smoother andless metal particles were ejected. The gas flux was set at anintermediate value of 5 l/min (too high fluxes disturbed the liquid bathand too low fluxes did not protect against oxidation). The best gasinjection angle was found to be about 30° from the sample surface plane(FIG. 2 b).

The surface cleanness is important for a high quality weld. Dirty andoxidized surfaces produce bubbles in the welds. Washing with water andethanol followed by ultrasonic cleaning gave sufficient surfacecleanness to produce good welds. Laser-cleaning prior to welding wasalso tried. A Q-Switched YAG laser was used to clean some samples as analternative to traditional cleaning with excellent results.

Various process parameters have been considered in order to obtain soundweldings by the dual beam method. The welding speed was fixed at 60 mm/swith 1700 W CO₂ power. In other arrangements welding speeds of 100 mm/sand more are possible. The average YAG laser power was fixed at 1200 W,and the following range of process parameters were investigated:

-   YAG energy and frequency: 4/300, 6/200, 8/150, 10/120, and 12/100    (J/Hz)-   YAG pulse length: 0.2, 1, and 2 ms.-   Distance between sources: 0, 1, 2, 3, 4, 5, and 6 mm.

Among the above process variables, the best conditions for crack-freewelds at 60 mm/s weld speed were obtained when the YAG beam was placed 3mm behind the CO₂ beam. Eight joules and 2 ms were the best compromisebetween an insufficient thermal transfer (4-6 J and/or 0.2-1 ms) andburning (12 J), but satisfactory results were also obtained using 10 J.Using 8 J pulse energy, and a frequency of 150 Hz, the intensity perpulse was about 30 W/cm². In comparison with single beam CO₂ welding seeFIG. 3 a, where there is a crack C.

The dual beam method produces an enlarged liquid bath. The result of thedual beam method is shown in FIG. 3 b, where there is no crack at all.Bubbles in the weld disappeared when the dual laser method was appliedas length of the liquid bath (L) was increased, thus increasing the timefor bubbles to rise to the surface.

FIG. 4 shows the temperature-time curve for a welding process with asingle laser beam (curve a) and the temperature-time curve for a weldingprocess with a dual laser beam (curve b) with a welding processaccording to the invention. In other words FIG. 4 shows the thermalhistory (a) and the cooling rate (b) during welding for single laserbeam (curve a) and dual laser beam (curve b) experiments. The peaktemperature was unaffected by the use of the second source, but thecooling rate in the critical location was substantially reduced, from2600° C./s to 1500° C./s (curve b).

The results also showed an increase in the pool length at 8 J, whichcorresponds to the best welding condition. At low power levels of thesecond beam, the liquid pool length (L) and width (W) was only slightlychanged since the heat input was low. At high energy levels (above 10 J)the YAG laser interacted directly with the plasma over the weld, formedby the first laser, both laser beams acting as a single heat source.

As proposed by Clyne and Davies (T. W. Clyne and G. J. Davies, J.British Foundry 74 (1981) 65), two different solidification periods canbe considered: a free feeding time (FT), where the interdendriticspacing permits an unimpeded flux of liquid, and a constrained feedingtime (CT) where the dendritic bridging leads to an increasingunderpressure in the residual liquid. Here these two periods of timewere considered as the interval between 60 and 10% and 10 and 1% liquid,for FT and CT respectively. These characteristic times can be calculatedfrom the extension of the solidification interval giving at centerlinedivided by the scanning velocity, giving: Single source: FT = 0.01 s andCT = 0.005 s Process according to the invention: FT = 0.043 s and CT =0.005 s

The feeding time is four times greater in the welding process accordingto the invention than in the single source case, and the HCS accordingto Clyne is now 0.12 in comparison with 0.5 for the single sourceprocess. Therefore, the welding process according to the inventionreduces cracking by extending the time where the liquid can feed thegrowing solid.

The thermal gradient can be estimated with a semi-empirical thermalmodel. The results at the centerline show that the thermal gradient atthe beginning of mushy zone was reduced from 400 K/mm to 175 K/mm Thisdecrease has consequences on the microstructure, leading to equiaxeddendrites near the centerline.

To prove the interest of the proposed laser welding process according tothe invention, a specimen was welded first with the single sourceproducing a longitudinal crack, and then the secondary laser wasswitched on during the experiment. Two steady regimes were observed: acracked weld when a single source was used and a crack-free weld whenthe second laser source was turned on. Few millimeters of YAGinteraction were required in the transient regime to close the crack(FIG. 5). The crack disappears after about 60 milliseconds of YAGinteraction in a sample where the YAG beam was turned on at the middleof the experiment (speed of the laser beams 60 mm/s, YAG energy 8J). Aseries of ten consecutive experiments confirmed the trend without asingle exception.

The difference between the laser welding process according to thepresent invention, and conventional welding processes is the use of twoor more locally and intensity wise well-controlled heat sources.Although this can be performed by any combination of available heatsources, as far as they are localized enough, it appears that lasertechnology has a major advantage over others methods because of the veryprecise control of spot size, position and heat input, essential to theeffectiveness of the present technique.

The effect of the dual laser system on strain rate can be discussed inthe following way: under the assumption of a fully constrained weld, themechanical stain rate is equal to the opposite of the thermally inducedstrain rate. This later value is proportional to the cooling rate, whichcan be controlled by the present welding method. Taking the values shownin FIG. 4, the strain rate at the critical location is decreased tonearly half of the value for conventional CO₂ welding by the use of thesecond laser source. The important point is that this second laser actsdirectly on the final part of the mushy zone, thus reducing the coolingrate most effectively, where it is needed.

It is possible that a dynamic correlation of the inter-source distanceand the other process parameters can also produce sound weldments.Moreover, any combination of heat sources, laser or otherwise, whichreproduce this process window could be used to avoid, cracks.

The new welding process avoids cracking in welding, in repair welding orin cladding of parts of metallic alloys which are sensitive to hotcracking. The process is using a first heat source 15, directed to theparts 11, 12 of the metallic alloy forming a melt pool 14 on the parts11, 12 of metal or metallic alloy. The heat source 15 and the parts 11,12 are moved relative to each other. The process is characterized inthat there is one 13 or more additional heat sources directed to theparts 11, 12 of metal or metallic alloy and following the first heatsource 15 in a distance and with substantially the same speed and in thesame direction as the first heat source 15. The additional heat source13 or heat sources are directed to the solidification region (mushyzone) 144 of the melt pool 14 generated by the first heat source 15. Thepower of the additional heat source 13 is set such as to reduce thelocal cooling rate of the solidification region 144 of the melt pool 14,or to even shortly reheat this region without substantial remelting orwith no remelting it at all and thereby reducing the tensile stresses oreven inducing compressive stresses. During this process a centralequiaxed zone might also be enhanced. By this new process the formationof hot cracks is avoided.

1. Process for avoiding cracking in welding, in repair welding or incladding of parts of metallic alloys (11, 12) which are sensitive to hotcracking, the process using a first heat source, directed to the partsof the metallic alloy and forming a melt pool (14) on the parts ofmetallic alloy, the heat source (15) and the parts being moved relativeto each other, the process being characterized in that there is one ormore second additional heat sources (13) directed to the parts (11, 12)of metallic alloy and following the first heat source (15) in a distanceand with substantially the same speed and in the same direction as thefirst heat source, the second heat source(s) (13) being directed to thesolidification region of the melt pool (14) generated by the first heatsource (15), the power of the additional heat source being set such asto reduce the local cooling rate of the solidification region (144) ofthe melt pool (14) or to even shortly reheat this region (144) withoutsubstantial remelting, thereby reducing the tensile stresses or eveninducing compressive stresses, thus avoiding formation of hot cracks. 2.Process as claimed in claim 1, one or more additional heat source (13)directed to the solidification region/the mushy zone (144) of the meltpool (14), the power of the additional heat source (13) being set suchas to reduce the local cooling rate or to even shortly reheat thisregion of the melt pool (14), without remelting at all or withoutsubstantial remelting, thereby reducing the tensile stresses or eveninducing compressive stresses, thus avoiding formation of hot cracks. 3.Process as claimed in claim 1, the first (15) and/or the additional heatsource(s) (13) being beams of high energy sources such as, lasersources, electron beam sources, electric arc sources, plasma sources, ora combination of those.
 4. Process as claimed in claim 1, the first (15)and/or the second (13) heat source being beams of one single or ofmultiple heat sources.
 5. Process as claimed in claim 1, using wire,powder or ribbons powder of an alloy of the same or of differentcomposition as the parts to be welded, for filling and claddingpurposes.
 6. Process as claimed in claim 1, using wire, powder, ribbonsor preapplied powder material of an alloy of the same or of differentcomposition as the parts to be cladded.
 7. Use of a process as claimedin claim 1, for avoiding cracking in welding, repair welding or claddingof parts of super alloys and/or steels.
 8. Use of a process as claimedin claim 1, for avoiding cracking in welding, repair welding or claddingof parts of aluminum alloys.